The present invention relates to turbomachines and, more particularly, to improved means for retaining rotor blades in rotor slots.
In the manufacture of bladed turbomachinery rotors, retention of the blades on a rotor, such as a turbine or fan rotor, has been commonly accomplished by insertion of metal strips within the rotor slot and subsequent bending of the ends of the metal strip such that the ends overlap the base of the blade and the radially extending faces of the rotor. These retainers have exhibited features which have proven to be undesirable. An example of one such undesirable feature is that loads tending to displace the rotor blade from its slot are absorbed by the bent ends of the metal strip. Since the metal strips have generally been designed to easily accommodate bending upon installation, they are susceptible to bending in opposite directions due to forces applied by the rotor blade.
Other undesirable features are associated with servicing of the rotor blade. Removal of the prior art devices during routine maintenance has proven troublesome since access, which in many instances cannot be conveniently provided, is required at both sides of the rotor disk. Furthermore, the prior art devices as described above are not suitable for re-use because of alterations in the physical and metallurgical properties associated with repeated bending of the ends of the retainers upon removal and reinstallation of the blades. The attendant replacement of the retainers is costly, especially if a substantial number of blades must be removed for servicing.
Other prior art devices, such as that shown in U.S. Pat. No. 3,936,234, have served generally to advance the state of the art in blade locking and retaining devices. However, while such a device provides excellent blade retention capacity with respect to the rotor disk, evidence has shown that a small amount of tangential freedom of the blade within the rotor disk slot permits relative motion to occur between the blade dovetail and the rotor disk during periods of windmilling rotation of the gas turbine engines. These periods may be as long as eight to twelve hours per day, and the resulting blade motion results in wear of a protective coating which is normally applied to the rotor disk slots. Loss of the coating permits galling wear of the disk dovetail pressure face. Repair of the disk dovetails is inconvenient, expensive, and can only be repeated a few times before the rotor disk becomes unserviceable.
The prior art device of U.S. Pat. No. 3,936,234 includes a biasing wedge comprised of a resilient material capable of elastic deformation which is inserted into a recess between the blade retainer and the bottom of the rotor disk slot to produce a radially outwardly directed load on the blade through compression of the biasing material. In the design of this prior art device, the elastic material should preferably have a low stiffness in order to permit compression over a tolerance range of slot gaps, and the material should be soft in order to prevent physical damage to the rotor disk slot bottom when the retainer spacer is installed. Current practice is to utilize an organic polymeric material such as nylon 6/6 in compression to provide the radial loading. The radial load that can be generated from a nylon-type of polymer is limited by the driving force required to install the retainer spacer. The use of soft compressive, non-metallic (or low strength) materials permits material creep at ambient temperatures and, combined with the radial compression that generates the loading, allows a minute amount of dovetail wear to loosen the system and reduce its effectiveness.
It might appear, therefore, that it would be desirable to provide the current prior art device with a hard, metallic, high-strength insert. However, such a combination: would inevitably damage the disk slot bottom during installation if, in fact, the insert could be driven into place; would be unable to compensate for slight differences in dovetail manufacturing tolerances; and, would be very difficult to remove.